Frequency conversion circuits



Patented 23, 1939 PATENT "OFFICE FREQUENGY CONVERSION CIRCUITS Ralph L. Miller, Bloomfield, N. 1., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application July 31, 1937, Serial N0.

156,698. Divided and this application April 30, 1938, Serial No'. 205,224

' This application is a division of my copending application Serial No. 156,698, filed July 31, 1937.

The invention relates to the production of alternating current waves by frequency conversion .5. methods.

An object of the invention is to convert alternating current waves of one'frequency into alternating current .waves of 'a different ire-- quency.

A more specific object is to produce alternating current waves of :desired frequencies which are accurate fractions of a given base frequency. 1 These objects are attained in accordance with the invention by-frequency conversion circuits utilizing a process which may be termed regenerative modulation. Regenerative modulation is produced in general byfeeding back the output of a balanced type modulator to the balanced or conjugate input thereof through a selective network, such as a filter, and an amplifier of fixed gain mu. Such a circuit is stable'and of av non-oscillatory nature as-long as the loss due to the balanced condition and the network or filter is greater than the gain mu of the' ampli- 5 fier. l g

The frequency conversion circuits of the invention employing this process, to be described hereinafter, may be used to produce electrical waves which are exact fractional ratios of a given 4 frequency applied to theinput, and which will follow amplitude and frequency variations in the applied waves over quite wide ranges, these circuits having exceptional frequency stability and efllciency in operation.

The various features and advantages of the circuits of the invention will be brought out in the following detailed description thereof when read in connection with the accompanying drawing of which:

Fig. 1 shows schematically a circult'which illustrates the basio principles of the invention;

Fig. 1A shows a modification of this basic circuit; and I Figs. 2 and 3 show schematically in greater detail frequency conversion circuits embodying different modifications of the invention.

The fundamental concept of regenerative modulation as applied to frequency conversion may be described by referring to Fig. 1. f

The frequency converter circuit of Fig. 1 includes a balanced. modulator I of the second order type, such as that disclosed in F, A, Cowan Patent No. 1,959,459, consisting of four copperoxide rectifier units, connected in ,a Wheatstone V Bo bridge formation, an input transformer 2 and f.. 4Claims. (01.250-36) an output transformer 3, the secondary winding of the input transformer and the primarywindingof the output transformer being connected in shunt with one diagonal 4, 5 of the bridgeanda source of modulating current being connected to the other conjugate diagonal 6, I of the bridge. The copper-oxide rectifier units 8, 9 are poled so that each is conductive in the directiontoward the common terminal 6, and the other rectifier units ll, ii are poled so that each is conductive away from the common terminal I. The primary winding of the input transformer 2 is connectedto a source (not shown) of the base frequency h to be converted, and a filter I2 is connected between the secondary winding of the output transformer 3 and the output terminal of the circuit. The source of modulating waves applied to the conjugate terminals 6, 1 of the modulator bridge is the regenerative circuit [3 comprising the filter network "I! land the one-way amplifier.

ll of gain mu having its input connected across the output of filter l2 and its output connected acrossathe conjugate terminals 6, l of the bridge I through the transformer IS. The gain muof the amplifier it in the regenerative circuit is selected so as to provide the required stability.

{is indicated,,a frequency multiplier [6, indicated. by the box so labeled (in Fig. 1A, would be included in the regenerative circuit i3 where it is ,desired to secure a fractional frequency having a denominator larger than 2.

Nowifithe input frequency ii is applied to the input of the second order modulator in the system of Fig. 1 and any frequency f2 in the output thereof is selected by the filter l2 and fed back through the amplifier It and transformer ii 'to the conjugate terminals 6, l of the modulator,

the two frequencies f1 and I: will combine in the modulator to produce the two side-band fre-' quencies fie-J which are at a certain loss with respect to 12. If the amplification mu is greater than the side-band loss plus the loss provided by the fllten if, the side-band frequencies will be fed back and will be applied to the conjugate terminals 8, 9 of the modulator I but at a greater amplitude. If the following arbitrary case 'is set up f ifg=fz which is the case where f: will sustain itself, it will be found-that so that the wave appearing in the output of the frequency conversion circuit is half the frequency;

I of the wave applied to the input thereof from the base frequency source, or

indicated.

In the general case for this type of circuit depending upon the order of modulation used nfrzi: a=fz or I (4) where n and m are integers depending upon the order of modulation. For the case of third order modulation, where:a third order instead of a secand order modulator is used,

n=1, m=2 or n=2, m=1

and

Ih'the case where the frequency multiplier I6 of Fig. 1A is used in the regenerative circuit I3, the following equations may be set up:

Fatima (7) s=m (8) where I: is the frequency in the output of the multiplier and r is the factor by which the feedback frequency f: is multiplied.

Solving these two equations simultaneously ves n a=i' rn fg i f (10) If the input wave is represented by the equation - Of =A cos wt (11) and the frequency component f: at the input of the modulator is represented as then the two side-band outputs which are obtained are given by In general, the frequency of interest is the frequency Y the other frequency being eliminated by the filter I2 in the regenerative circuit. Since the input to the modulator from the regenerative circuit. is obtained from the side-band output in the case of a self-sustaining wave, then the condition for the phase relation of the regenerative component at the input and output of the modulator is amplifier, less the loss of the filter network. In.

- general, the amplitude of the sub-harmonic will I be a function of the fundamental, although it may not be a linear relation.

In frequency conversion circuits employing regenerative modulation, the amplifier merely acts as a means of supplying the necessary gain in the feedback circuit and does not become overloaded unless an extremely large input is applied to the regenerative modulator. Because of this fact, it follows that if several regenerative modulator circuits are to be used at different frequency ranges, it should bepossible to utilize the same amplifier for each of the circuits, using selective networks or other means to separate the frequencies on the inputs and outputs.

A special application of this principle is in the case where several regenerative modulators are to be used in cascade. In the case of the use of second order regenerative modulators, each stage -will reduce the frequency applied thereto by a factor of two. By proper choice of the selective networks, the same amplifier may be used for several stages. Fig. 2'shows the circuit diagram of such an arrangement which was designed to convert a. 2400 cycle frequency into 300 cycles and the intermediate frequencies 1200 and 600 cycles, which arrangement employs three stages of regenerative modulation and the same amplifier for each stage.

Referring to Fig. 2, it will be seen that the regenerative modulators in the three stages are of the double balanced type, such as is disclosed in Cowan Patent No. 2,025,158, each employing a copper-oxide rectifier bridge with all the copperoxide rectifier elements poled in the same direction, and an input and an output transformer connected respectively across the two diagonals of the bridge. The first stage of the circuit comprises a modulator including the copper-oxide rectifier bridge I38, the input transformer I 39 having its primary winding connected to the source of alternating current waves of the frequency (2400 cycles) to be converted and its secondary winding connected across one diagonal of the bridge, and the output transformer I40 having its primary winding connected across the other diagonal of the bridge I 38, a single pentode amplifying vacuum tube I4I including in its control grid-cathode circuit the secondary winding of modulation loss is equal to the gain mu of the transformer I40, the combined output, feedback The second stage of the circuit of Fig. 2 comprises a modulator including the' copper-oxide rectifier bridge I41, similar to the bridge I55, the transformer I42 having its secondary winding 5 connected across one diagonal of the bridge I41 through the circuit I44 and the series resistances I45 and I49 and the modulator output transformer I50 having its primary winding connected across the other diagonal of bridge I41, the am- 1 plifying vacuum tube I4I including in its control grid-cathode circuit the secondary winding of transformer I50 in series with the secondary winding of the transformer I40, the combined output, interstage and feedback transformer I 5i having its primary winding connected in the anode-cathode circuit of the amplifier tube I in series with the primary winding of transformer I42, the anti-resonant circuit I52 shunting that winding and the feedback circuit I56 connecting the secondary winding of transformer I5I across the mid-point of the secondary winding of transformer I42 and the mid-point of the primary of transformer I50 through the series resistances I54 and I55.

The third stage of the freqimicy conversion circuit of Fig. 2 comprises the modulator including the copper-oxide rectifier bridge I55, similar to the bridges I55 and I41, the transformer I5l having itssecondary winding connected across one diagonal of bridge I56, through the series resistances I51 and I55, the transformer I55 having its primary winding connected across the other diagonal of the bridge I55, the amplifying vacuum tube I4I including in its control grid-cathode s5 circuit the secondary winding of transformer I55 in series with the secondary windings of transformers I55 and I40. the combined output and feedback transformer I50 having its primary winding connected in the anode-cathode circuit 40 of amplifier tube I in series with the primary windings of the transformers I5I and I42, the anti-resonant circuit I 6| shunting that winding, and the feedback circuit I52 connecting the secondary winding of transformer I55 across the mid-point of thesecondary winding of transformer I5I and the midpoint of the primary winding of transformer I56, through the series resistances I65 and I64.

The anti-resonant circuits I45, I52 and I" are respectively tuned to the successively lower sub- 1 multiple frequencies to be fed back to the conjugate terminals of the modulator in each stage through the feedback circuits I44, I and I62,

- -respectively, to reduce the input frequency applied to the primary winding of the modulator input transformer in the succeeding stages by a factor of two in each case.

. In the case illustrated, where the wave applied to the input of the frequency conversion circuit has a frequency of 2400 cycles, the anti-resonant circuit I43 will be tuned to select the frequency 1200 cycles from the waves in the output of amplifier HI, and this frequency will be fed back to 55 the conjugate terminals of the modulator in the first stage through transformer I42 and circuit I44, and will be applied also through resistances I45, I46 to the input of the modulator in the next stage.

70 The anti-resonant circuit I52 is tuned to select the frequency 600 cycles from the combinationwaves in the output of amplifier I, which frequency will be fed back to the conlugate input .of the modulator in the second stage through 75 transformer I5I and circuit I55 and will be applied also through resistances I51 and I55 applied to the input of the modulator in the third stage.

The resonant circuit I5I in the third stage is tuned to select the frequency 300 cycles which frequency will be fed back through transformer I55 5 and circuit I52 to the conjugate input of the modulator in the third stage. The amplifier tube I will operate in each of the three stages to amplify first the applied frequencies and then the combination frequencies produced by the 10 modulator by regeneration in each stage.

Outgoing circuits are connected to the secondary winding of transformers I42, I5I and I60 for respectively taking of! the sub-multiple frequencies produced in these windings for each stage of sub-multiplication, 1200 cycles, 600 cycles and 300 cycles, as indicated.

In the circuit of Fig. 2 the balance of the modulator unit alone is utilized to prevent oscillation, whereas in each of the succeeding stages the bal- 2o ance of the modulator unit plus the loss of the resonant circuit in the output of the amplifier in each regenerative path is depended on-to prevent omllations in these paths. The tendency to oscillate increases as the number of stages in- 25 creases. A lesser degree of balance will be required if a resonant circuit or filter tuned to the same frequency as the resonant circuit in the output of the amplifier in each stage is included in the input circuit of the amplifier for each 80 stage. If filters in the input circuits of the amplifier are employed, the only limit to the number of stages which can be used depends o the load carrying capacity of the amplifier and its ability to furnish the proper gain for each fre- 86 I quency.

In Fig. 3 is shown a frequency conversion circuit employinga single amplifier for a plurality of stages of regenerative modulation as in the system of Fig. 2. and the same modulator for the 40 several stages and other simplifications which will be pointed out below. I

The first regenerative stage in the conversion circuit of Fig. 3 comprising a modulator of the double-balanced type similar to those used in 45 the system of Fig. 2, comprising the copper-oxide rectifier bridge I having one diagonal connected across the source of alternating current of the base frequency f to be converted (not shown) through the potentiometer m, a regenerative circuit comprising the single amplifying pentode vacuum tube I51 and the combination tuned feedback and output transformer I55. The control grid-cathode circuit of the amplifier I51 is connected across the output diagonal of the modulator bridge I65 through the potentiometer I66. The anode-cathode circuit of the amplifying tube I61 includes the primary winding I10 of transformer I66, this winding being shunted by the tuning condenser Ill. A secondary (feedback) winding I12 of transformer I65 connects the output of the regenerative cir-. cuit across the mid-points of the potentiometers I65 and I66 connected respectively across the 65 input and output diagonals of rectifier bridge I55, in conjugate relation to the incoming circult and the input of amplifier I 61 in the input of the regenerative circuit. The transformer I66 has a third (output) winding I13 for takihg oil 70 the converted frequency in the output of the first regenerative modulator stage. A resistance pad I14 is inserted in the regenerative circuit be-. tween the feedback winding I12 of transformer I55 and the conjugate terminals of the modulator bridge I65, to adjust the loss in the feedback circuit to a suitable value.

The second stage of the conversion circuit of Fig. 3 comprising the same modulator rectifier bridge I65 and associated potentiometers I66, I69, and the same amplifier I61 as used in the first stage, but a separate combined interstage, output and feedback transformer I15 and a separate feedback circuit I16.

The primary winding I11 of transformer I15, shunted by a tuning condenser I18, is connected in the anode-cathode circuit of the amplifying vacuum tube I61 in series with the primary winding I10 of the first stage transformer I68, shunted by condenser I1I. The feedback circuit I16 of the second stage has its input connected to a second (feedback) winding I19 of transformer I15 and its output connected across the base frequency supply circuit in front of potentiometer I66. -The circuit I16 includes a resistance pad I80, similar to the pad I14 in the first stage regenerative circuit, for providing a desired amount of loss in the feedback circuit I16. A third (output) winding I 8| in transformer I15 is provided for taking off the converted frequency from the second stage.

Plate current is supplied to the plate of the amplifying tube I61 from battery I82 through winding I11 and I10of transformer I15 and I68, in series, a positive bias potential for the screen grid of tube I61 is also obtained from plate battery I82, and a suitable negative bias on the control grid of tube I61 is obtained by the usual parallel resistance-condenser biasing combination I83 in the control grid-cathode circuit of tube I61 through the potentiometer 'I 69. Heating current for the cathode of tube I61 may be supplied from any suitable source, not shown. I

The anti-resonant circuit comprising winding I10 and parallel condenser "I in the output of tube I61 is tuned to the sub-multiple frequency which will be applied to the second stage of regenerative modulation comprising the same modulator I65,.the same amplifier I61, and the regenerative circuit I16 to produce a sustained wave of the frequency which will be induced in the output winding I8I of transformer I15 connected to the load circuit, and a sustained wave of the sub-multiple frequency will be induced in the other load circuit connected to the winding I13 of transformer I68.

The pad I14 in the feedback circuit of the first regenerative modulator stage will be selected to provide the proper loss to make the operation of the first stage stable, and the pad I is likewise adjusted to make the second regenerative modulator stage stable.

It a larger amount of power in the produced waves of sub-multiple frequencies is required in the circuit of Figs. 2 and 3, two amplifying vacuum tubes may be used therein in place of the greater-step-down in frequency. The circuits of l Figs. 2 and 3 as illustrated and described employ regenerative modulators of a simple second order type, but it is apparent that the same I principles hold if other types of regenerative modulators are used.

Other modifications of the circuits illustrated and described above within the spirit and scope of the invention will occur to persons skilled in theart.

What is claimed is: i

1. A frequency conversion circuit comprising a back to one of the conjugate inputs of said modulator a wave derived from the output thereof to modulate in the modulator with waves supplied to the other input thereof, the same amplifier being employed in all of said stages, means for impressing a wave of a given frequency on the other input of the modulator in the first stage, a plurality of selective networks in series in the output of said amplifier each tuned to a different frequency bearing a definite relation to said given frequency, the regenerative circuit in each stage including a different one of said networks, the other input of the modulator in each stage after the first being taken from the selective network in the preceding stage, and a utilization circuit for picking off a converted frequency, connected to the output of one or more stages.

2. A frequency conversion circuit comprising a plurality of stages of regenerative modulation each consisting of a balanced modulator having two conjugately connected inputs, and a feedback circuit including an amplifier, connecting the output of the modulator with one input thereof, the same amplifier being employed in the feedback circuit of all the stages, means for impressing a wave of a given base frequency on the other input of the modulator in the first stage, a plurality of selective networks connected in the output of the amplifier each tuned to select a different frequency which is a desired fraction of said given frequency, a different one of said networks being included in the feedback circuit of each stage, the input to the modulator in each stage after the first being taken off from the selective network in the preceding stage, and a utilization circuit coupled to the selective network in one of the stages for taking off a sustained wave of the selected frequency.

3. A frequency conversion circuit comprising a plurality of stages of regenerative modulation each consisting of a balanced modulator havin two conjugately connected inputs, and a circuit including an amplifier for feeding back to one of the conjugate inputs of the modulator a wave derived from the output thereof, the same modulator and the same amplifier being employed in all the stages, means for impressing a wave of a given frequency on the other input of the modulator in the first stage, and a plurality of selective networks in series in the output of said amplifier, each tuned to a different frequency bearing a definite relation to said given frequency, the feedback circuit in each stage including a different one of said networks, the input of the modulator for each stage of regeneration after the first being taken from the selective network in the preceding stage, and a utilization circuit for picking oil? a converted frequency from the selective network in at least one of said stages.

4. The frequency conversion circuit of claim 3, in which said balanced modulator comprises a plurality of non-linear modulating elements connected in a balanced bridge circuit, and two resistances respectively connected across the two diagonals of said bridge circuit, said wave of given frequency being impressed on one diagonal of said bridge circuit across one of said resistances, the other diagonal of said bridge circuit being connected to the input of said amplifier across the other resistance, the output of the feedback circuit in successive stages being connected across the mid-points of the two resistances and across said one resistance, respectively.

RALPH L. MILLER. 

